The present invention relates to an X-ray tube device which can produce X-ray focal spots of generally the same shape and of any size required in accordance with the type and size of a body that is examined and can produce a tube current of any required magnitude in correspondence to the size of the X-ray focal spot.
Generally, X-ray tube devices are used in medical applications such as X-ray diagnosis, for example, The devices used conventionally for examination of the stomach, etc. are rotating anode X-ray tubes. In an X-ray tube such as this, a cathode assembly and a disk shaped anode target are offset from the tube axis and face one another in an evacuated envelope and the anode target is arranged so that it is rotated by a rotor which is rotatably driven by electromagnetic induction produced by a stator.
The cathode assembly has a structure such as shown in FIG. 10 in which a cathode filament 11 is disposed in a focussing dimple 16 of a focussing electrode 12. Cathode filament 11 is constituted by a tungsten coil so that it may emit thermoelectrons, and these thermoelectrons are focussed by electric field formed by cathode filament 11 and focussing electrode 12 being brought to the same potential in order to achieve this. In the figure, dashed lines 13 represent the equipotential curves in the vicinity of focussing electrode 12, 14 the loci of electrons emitted from a generally central portion of cathode filament 11 and 15 the loci of electrons emitted from locations near the side surfaces of cathode filament 11.
Cathode filament 11 is generally used in a temperature limited region in the above prior art cathode assembly, and so in order to increase the field intensity in the vicinity of cathode filament 11, a portion of the cathode is protruded into the focussing dimple 16. As a result, the equipotential plane in the vicinity of cathode filament 11 takes a form that bulges at the centre of cathode filament 11, as indicated by dashed line 13a, and electrons 15 emitted from the substantially side walls of cathode filament 11 are directed sideways. These electrons 15 are not focussed in the same direction as electrons 14 that are emitted from the substantially central portion of cathode filament 11 and are directed forwards but, as shown in FIG. 10, have loci that intersect on the axis. Therefore, the electron intensity distribution at a surface of the anode target 13 is not uniform, showing for example twin peaks in FIG. 10.
Since it is thus not possible for the focussing electrode 12 to effect satisfactorily tight focussing of electrons emitted from cathode filament 11, achieving a small X-ray focal spot at the location of anode target 13 necessitates use of a small cathode. This in turn means that there is a problem of cathode filament 11 reliability, since it is not possible to produce electrons at satisfactorily high density without raising the cathode temperature.
Further, it is not possible to produce a very fine X-ray focal spot since electrons are not all advancing in the same direction at the location of anode target 13. Thus, since there is no sharpness in the electron distribution and it is not possible to achieve a required electron distribution, it is impossible to achieve a satisfactorily high degree of spacial resolution of X-ray image obtained by using these conventional X-ray tubes. Also, it is not possible to simultaneously satisfy the two requirements X-ray focal spot is made smaller by lowering the maximum temperature rise caused by impingement of electrons into small area on anode target 13 and that the amount of incident electrons are increased. As they constitute an obstacle to improvement of spacial resolution and reduction of photon noise, these facts prevent production of a satisfactorily clear image in production of a projected image by X-rays emitted from anode target 13.
A method one can think of for eliminating this drawback is to use a cathode filament in the form of a flat plate.
An example of this is the proposal disclosed in Japanese Laid-open Patent Application No. 55-68056.
To describe the example of prior art that is shown in FIG. 11 and employs such a cathode constituted by a strip-like flat plate 21 is a cathode filament which is constituted by a strip-like flat plate and formed into a shape .pi. and which is mounted on filament support posts (not shown) and is directly heated and emits thermoelectrons on connection of power. 22 is a focussing electrode which has a shallow focussing dimple depth (H) and serves to focus electrons that exist from cathode filament 21. 23 indicates equipotential curves in the vicinity of focussing electrode 22. An anode target, indicated by the reference numeral 28, is maintained at a high positive potential with respect to cathode filament 21 and focussing electrode 22 and is located at a point that is coincident with the focal distance of the electron lens constituted by the focussing electrode.
However, this prior art example has the following drawbacks.
The loci of electrons 25 exciting from side surfaces of cathode filament 21 and of electrons 24 exciting from the central portion are very different and the electron distribution 27 on anode target 28 has secondary X-ray focal spots as indicated in FIG. 11. The reason for this is that the loci of electrons exiting from end portions of cathode filament 21 constituted by a strip-like flat plate is as indicated by the line 29 in FIG. 13. Dashed curve 30 indicates the equipotential curve at a location that is very close to the surface of cathode filament 21 and, as seen in the figure, it has a distribution which sags in the gaps 31 between the end portions of cathode filament 21 and focussing electrode 22, so producing local concave lenses. As a result, the loci 29 of electrons emitted from locations near to the end portions of cathode filament 21 are closer to the walls of focussing electrode 22 than they would be if the equipotential curve 30 were uniform. Further, the equipotential curves 23 in focussing electrode 22 are more curved near the walls of focussing electrode 22 than in the central portion of focussing electrode 22, so resulting in aberration, since the focal distance with loci 29 is shorter than it is with 24, and it is thus not possible to achieve a satisfactory degree of focussing. When the value of current becomes larger, electron beam distribution width on the target surface is changed to a larger value than in small current case, because of space charge effects.
When the focussing electrode is brought to the same potential as the filament and, in order to achieve a greater focussing effect, the focussing electrode 22 depth H is made large and f is made small keeping the anode to cathode distance the same, the field in the vicinity of cathode filament 21 becomes weaker, so resulting in a space charge limited state and variation in the value of current depending on the anode potential. In some cases it is not possible to get a current of more than 10 mA when the anode voltage is of the order of 30 kV.
There is an example of a structure in which a bias voltage that is positive with respect to cathode filament 21 is applied to focussing electrode 22 or on an electrode with a shallow focussing dimple that is located a little forward of this, but one can expect this to result in poor electron beam focussability in the direction of cathode filament (in the direction normal to the plane of FIG. 11). Basically, therefore, the abovenoted disclosed and published art gives no indication of how to make it practically possible to freely change the width of electron beam on the anode target while maintaining a similar beam width ratio in axial and transversal directions.
In this example, one would anticipate that it is not possible to change the size of an X-ray focal spot while maintaining a similar shape unless bias voltages of different values are applied independently going in the direction of length and the direction of width of the cathode filament as taught in another example, Japanese Laid-open Patent Application No. 59-94348.
A conventional example of means for producing X-ray focal spots of different sizes while keeping the X-ray focal spot shape almost constant is that disclosed in Japanese Laid-open Patent Application No. 59-94348 in which independent voltages are applied in two directions that cross at right-angles and correspond to directions going along the length and the width of the X-ray focal spot. This means has a construction such as shown in FIG. 12 by way of example namely separated two pairs of electrodes so that, individual voltages are applied to upper and lower electrodes 33 and left and right electrodes 34 surrounding a rectangular flat filament electrode 21. Producing a required X-ray focal spot in this example necessitates imposition of individual voltages in the direction of length (up to down) and in the direction of width (left to right). The X-ray tube construction is therefore complex, a greater number of high voltage cable core strands is needed and selection of requisite voltages in use of the equipment is difficult. Further, it is not possible to get a X-ray focal spot with sharp edges in this example, because of electrons from side surfaces of the cathode as described earlier. Also, because of the field at electrode corner portions, changes in bias voltage are accompanied by changes in the shape of corner portions of the X-ray focal spot.